The --bwlimit option in rsync allows you to limit the bandwidth used by the transfer. You can specify the bandwidth limit in kilobytes per second.
For example, to limit the transfer speed to 128 kilobytes per second, use the following command:
rsync --bwlimit=128 <source> <destination>
where <source> and <destination> are the paths to the source and destination directories.
Note that the --bwlimit option is only available in rsync version 3.0 or later.